Acoustic resonators that are formed of thin films may be used in a number of applications that require a precisely controlled frequency. A Thin Film Bulk Acoustic Resonator (FBAR) or a Stacked Thin Film Bulk Acoustic Resonator (SBAR) may be used as a filter in a cellular telephone or other device in which size, cost and frequency stability are important factors.
An FBAR includes a thin film of piezoelectric material between two conductive electrodes, while an SBAR includes additional layers of piezoelectric material, with each such layer separating two electrodes. While solidly mounted resonators are known, the active layers of an FBAR or SBAR are often suspended in air by supporting the layers around the perimeter. The air/resonator interfaces at both sides of the stack of layers partially trap the energy that is generated during operation.
When a time-varying electrical field is created by applying a signal across two electrodes that are separated by a piezoelectric layer, the piezoelectric material converts some of the electrical energy into mechanical energy in the form of sound waves. The sound waves propagate in the same direction as the electrical field and are reflected at the air/resonator interfaces. For a properly fabricated FBAR or SBAR, the sound waves will have a particular mechanical resonance.
As mentioned above, an FBAR or SBAR can be used as a filter, since it will function as an electronic resonator when allowed to operate at its mechanical resonant frequency. At this mechanical resonant frequency, the half wavelength of the sound waves propagating through the resonator is approximately equal to the total thickness of the resonator for a given phase velocity of sound in the piezoelectric material. Acoustic resonators may be used alone or in combination. For example, a bandpass filter is formed by electrically connecting several resonators to provide a desired filter response. Several filter topologies are possible. One favored topology is the half-ladder topology, where a group of resonators are connected in series (series resonators) and in between the series resonators are shunt resonators that are connected to ground. The series resonators are fabricated such that their resonant frequency is approximately 3% higher than the shunt resonators. Since the thickness of the piezoelectric layer can be the same for the series and shunt resonators, the piezoelectric deposition is often “shared” between resonators.
It becomes manifest that an important characteristic of acoustic resonators is an ability to maintain resonance. This has proved problematic when acoustic resonators are placed in an environment that undergoes temperature fluctuations, since a frequency shift (Δf) will occur if a variation in temperature (ΔT) induces a change in the thickness (Δt) and/or wave velocity (ΔV) for one or more layers of a resonator. Specifically, the resonant frequency f0 and the temperature coefficient of frequency are respectively defined as follows:f0=V/2t0  (1)Δf/f0=ΔV/V−Δt/t0  (2)where V is the velocity of the acoustic wave propagating through the acoustic resonator and t0 is the thickness of the resonator. The thickness is defined in terms of the acoustic wavelength as follows:t0=λ/2  (3)where λ is the wavelength of the acoustic wave in the medium through which it propagates. In the materials employed to fabricate acoustic resonators, the thickness, t0, usually increases with positive changes in temperature, ΔT. On the other hand, the velocity of wave propagation through the materials usually decreases with positive changes in temperature. These two factors combine to provide the phenomenon that is referred to as negative temperature coefficient of frequency. From equations (1) and (2) it is seen that the resonant frequency, f0, of an acoustic resonator generally decreases as the temperature increases. This fluctuation in resonance is often an undesirable characteristic.
One known approach to compensating for the variations in temperature is to incorporate a frequency stabilization circuit. However, space restrictions of cellular phones and similar devices impose limitations on the use of auxiliary circuits. Another approach is described in a paper entitled “Thin Film Resonators and Filters” by K. M. Lakin, 1999 IEEE Ultrasonics Symposium, Jun. 1, 1999. This second approach applies to solidly mounted resonators (SMRs), which are mounted along a supporting surface, rather than being suspended by peripheral support from the supporting surface. Acoustic isolation being an SMR and the substrate in which it is formed is achieved by forming a reflector (typically a Bragg reflector) between the SMR and substrate. The reflector is a layer stack having alternating layers of high index and low index materials, with each layer having a thickness of approximately one-quarter wavelength of the resonant frequency of the SMR. According to the second approach, if silicon dioxide (SiO2) is used to form one of the index-specific layers, a degree of temperature compensation will occur as a result of the temperature coefficient of SiO2. However, a drawback is that SiO2 is hydrophilic, so that the performance of the SMR may degrade in a humid atmosphere. Another concern is that the level of compensation is partially determined by the target resonant frequency, since the SiO2 is formed as one-quarter wavelength layers.
What is needed is an acoustic resonator and method of using the same that maintains resonance when subjected to variations in temperature.